Zika virus
infection 2015-16 Epidemic
September 6, 2016
Update
Paul Herscu ND,
MPH
Herscu Laboratory
Zika Virus Disease
And Targeted Vector Control ‘The Future is Already Here — Just Not Evenly
Distributed’
This article addresses only one aspect of prevention of Zika virus caused disease; it is the fifth in a series. For previous writings on the subject and to contextualize this writing, please refer to my blog.
I had previously written
on the vector control benefits of attempting ‘functional’ extinction of the Aedes
aegypti species that likely carries Yellow fever, Dengue fever, Chikungunya,
and Zika virus, illnesses that lead to morbidity or mortality in the many
thousands every year. The arguments were many, but the main point was that Zika
virus was absolutely going to impact the health of Americans via local
transmission and that many would suffer needlessly. I went on to urge that the
time was right for the planned ‘functional’ extinction of Aedes aegypti
but at this point that approach would no longer be sufficient. As expected, we
have since seen the first infant death
in the U.S., due to Zika virus and have the first U.S. birth of a child with
Zika virus caused microcephaly. More that 2,722 people in the U.S. have been
infected, with 35 from local transmission, according to the CDC Case Counts
in the U.S. Sadly, more will likely follow.
In the face of this tragedy, there are essentially three
possible strategies: precautionary avoidance of being bitten, immunizations,
and vector control. Much work, money, energy, and press is given to the topic
of immunization. Many of us believe that the vector control side of the
equation has been mostly relegated to the sidelines. I would like to highlight
a more rational and reasonable view of vector control, for the time that we
live in now, with the extent of mosquito populations there are now, in the
beginning of September, 2016.
The second part of the title of this writing is partially
a quote, often attributed to William Gibson, and goes right to the heart of the
matter. The nature of vector control, of how to control the mosquitoes, is at a
time of significant transition. We can briefly separate vector control into two
prevailing forces: targeted vector control versus widespread vector control.
Historically, a great deal of vector control is widespread in nature, a blunt
instrument, not choosing a single species as a target. One common control
example is spraying insecticides for mosquitoes. The hope is that this will
knock down the overall mosquito population and thereby limit transmission of
disease, here the Zika virus. While this may work, it also kills many species
of mosquitoes that do not bite people. And worse, it kills many other insects, aside
from mosquitoes, such as bees. As well, we
have a century of experience to understand that pesticides accelerate
adaptation pressures towards the toxin, making insects immune. There is
a better way: targeted controls!
I would like to start with an illustration from another
field for a moment. Historically, the most common form of chemotherapy is a
widespread attempt to interrupt the life cycle of fast growing cells, such as
cancers. Unfortunately there are other fast growing cells in the body, such as
hair, and mucus membranes, and this blunted approach causes ulceration,
mucositis, and hair loss as side effects to these agents. Those types of
chemotherapies cannot differentiate between ‘good’ cells or ‘bad’ cells,
working rather on all fast growing cells. More recently, though, we have
developed, in precision medicine, targeted therapies; treatments aimed at the
cancer cells while sparing many non-cancer cells, targeting specific genes or
proteins that are highly associated with that cancer. Monoclonal antibodies are
one such example.
It may be that, as compared to vector control, the future
is here in cancer care. It may be that when working on vector controls we might
become better stewards of the environment by targeting the threatening organism
rather than wholesale destruction.
In the last piece I highlighted one targeted therapy
aimed at Aedes aegypti that may have worked well at the time it was
written, in April 2016, trying to stem the tide, limit the number of species
that may carry the Zika virus and at the same time sparing all other mosquito
and insect species. I would like to highlight a different method that may work
at this time.
I think it is time to get into a bacterial warfare with
the Zika virus, using Wolbachia species
as the main tool, and here are the reasons why:
1. There are numerous studies showing that Wolbachia in
insects impacts the likelihood of transmission of diseases, including limiting the spread
of Zika virus.
2. Wolbachia is an endosymbiont in arthropods, living in over half of the insects alive, so it is already present in nature.
3. Introducing Wolbachia into a new species of insect is not a difficult task, and has been accomplished in the lab easily enough.
4. Introduction of Wolbachia into a new species is essentially a one-time event as the process self propagates, making this form of control relatively inexpensive.
5. As we watch Wolbachia spread from insect species to insect species, it seems plausible to believe that this will happen in Aedes aegypti, sooner or later, and that all we are doing is speeding up that time.
6. It is always better to work with nature, instead of fighting it. Here this introduction of bacteria into the mosquito is nature evolving not us trying to stop nature from doing what nature wants to do. A much more likely formula for success.
7. In fact, this form of vector control has begun in several countries, and seems to be succeeding. Overall Zika virus transmission is down AND we are not killing other insect species at the same time.
2. Wolbachia is an endosymbiont in arthropods, living in over half of the insects alive, so it is already present in nature.
3. Introducing Wolbachia into a new species of insect is not a difficult task, and has been accomplished in the lab easily enough.
4. Introduction of Wolbachia into a new species is essentially a one-time event as the process self propagates, making this form of control relatively inexpensive.
5. As we watch Wolbachia spread from insect species to insect species, it seems plausible to believe that this will happen in Aedes aegypti, sooner or later, and that all we are doing is speeding up that time.
6. It is always better to work with nature, instead of fighting it. Here this introduction of bacteria into the mosquito is nature evolving not us trying to stop nature from doing what nature wants to do. A much more likely formula for success.
7. In fact, this form of vector control has begun in several countries, and seems to be succeeding. Overall Zika virus transmission is down AND we are not killing other insect species at the same time.
It does not take a great stretch of the imagination to
develop a targeted vector control program, that includes a combination of
biological introduction of other organisms to control the viral transmission,
here I am offering up Wolbachia, in combination with targeted mosquito killing
methods, here I offer the Sterile Insect Technique (SIT) or RIDL (Release of
Insects Carrying Dominant Lethal), both described last time. This
type of program targets the one insect while leaving others relatively
unharmed, as opposed to spraying insecticides. In many ways WHO agrees with
this concept, though it is months delayed in execution.
This future has begun to take hold, even in the US. Clovis,
California has begun to release these mosquitoes that have had Wolbachia
introduced. In a way, this experiment is underway, though slow to take hold. In
the balance of risk of leaving the current state as it is and introducing these
types of targeted applications, I think the scales are easily tipped by the
screaming of infants and parents. If we do not take advantage of these tools
and strategies, the only choices left will be immunizations which have a
measured though incomplete success, and avoidance which has shown to be
problematic as a lone strategy.
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